1. Field of the Invention
The present invention relates to a lithium battery, and, more particularly, to a lithium battery using a non-aqueous solvent for extended charge/discharge cycle life, improved high-temperature storage stability and improved low-temperature discharge characteristics.
2. Description of the Related Art
Lithium batteries that are widely being used as power supplies for portable electronic devices such as notebook computers, camcorders or mobile phones, include a cathode containing a lithium metal composite oxide or sulfur capable of intercalating and deintercalating lithium ions, an anode containing a carbon material or lithium metal, and an electrolytic solution having an appropriate amount of a lithium salt dissolved in a non-aqueous mixed solvent.
The average discharge voltage of lithium batteries is in a range of about 3.6 to about 3.7 V, which is relatively higher than that of other alkali batteries or nickel-cadmium batteries. In order to attain such a high operating voltage, it is necessary to form an electrolytic solution that is electrochemically stable at a charge/discharge voltage area in a range of 0 to 4.2 V. To this end, a mixed solvent having a cyclic carbonate such as ethylene carbonate or propylene carbonate, and a linear carbonate such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate, mixed in each appropriate amount, is used as a solvent for the electrolytic solution. A lithium salt such as LiPF6, LiBF4 or LiClO4 is typically used as a solute for the electrolytic solution, and serves as a source of lithium ions in a cell, allowing the lithium battery to operate.
During an initial charge stage of a lithium battery, lithium ions in a cathode active material such as lithium metal composite oxide move to an anode active material such as graphite and are intercalated between lattice planes of the anode active material. Since lithium ions are highly reactive, the lithium ions in the electrolytic solution react with carbon of the anode active material such as graphite on the surface of the anode active material, thus producing compounds such as Li2CO3, Li2O, LiOH or the like. These compounds form a solid electrolyte interface (SEI) film on the surface of the anode active material such as graphite.
The SEI film works as an ion tunnel and allows only lithium ions to pass through. The SEI film working as an ion tunnel prevents a laminated structure of an anode from being disintegrated by intercalation of high molecular weight organic solvent molecules moving with lithium ions in the electrolytic solution to the anode active material. Thus, the electrolytic solution and the anode active material do not contact each other by the SEI film, so that the electrolytic solution is not decomposed by intercalation of high molecular weight organic solvent molecules and the amount of lithium ions in the electrolytic solution is reversibly maintained, thereby ensuring stable charge and discharge.
In a thin prismatic type battery, however, the thickness of the battery may increase during charging due to a gas such as CO, CO2, CH4, C2H6 or the like generated when a carbonate-based solvent is decomposed during formation of the SEI film, as described in C. R. Yang, Y. Y. Wang and C. C. Wan, J. Power Sources, pages 66–70, Vol. 72, 1998.
When a battery is stored at high temperature in a fully charged state, the SEI film slowly decays due to increased electrochemical energy and thermal energy. Thus, side reactions of an electrolytic solution at an exposed surface of and around an anode may continuously occur so that gas is continuously generated inside the battery, resulting in an increase in the internal pressure of the battery. Consequently, a prismatic type or pouch type battery may become bulky, impairing stability of a portable electronic device such as a mobile phone, a notebook type computer or the like, when stored at high temperature.
To suppress an increase in the internal pressure of a battery, research into techniques for changing phases in the reaction for forming an SEI film by adding an additive to an electrolytic solution, has been carried out. One example of such research is found in Japanese Patent Laid-Open Publication No. Hei 07-176323 disclosing an electrolytic solution having CO2 added thereto. Japanese Patent Laid-Open Publication No. Hei 07-320779 discloses a technique of suppressing decomposition of an electrolytic solution by adding a sulfide-based compound thereto. Japanese Patent Laid-Open Publication No. Hei 09-73918 discloses a battery having improved high-temperature storage stability using diphenyl picryl hydrazyl (DPPH). Japanese Patent Laid-Open Publication No. Hei 08-321313 discloses a battery having an improved charge/discharge cycle by adding a specific compound to an electrolytic solution.
To date, it has been known that a specific compound added to an electrolytic solution for the purpose of enhancing battery performance may improve some factors but may degrade other factors in view of battery performance.
A nonaqueous solvent contained in a conventional lithium battery employs a mixed solvent having a large amount of ethylene carbonate as a cyclic carbonate compound having a high dielectric constant and an appropriate amount of low-viscosity linear carbonate compound such as dimethyl carbonate or diethyl carbonate. For example, U.S. Pat. No. 5,686,138 discloses a lithium secondary battery using a nonaqueous solvent comprising 20 to 80% by volume of ethylene carbonate. In such a lithium battery comprising a large amount of ethylene carbonate, however, an SEI film is structurally unstable, resulting in a sharp increase of the internal pressure of the battery.
Since ethylene carbonate has a relatively high freezing point, i.e., 37 to 39° C., it is at a solid state at room temperature, meaning that its ion conductivity is low at low temperature. Thus, a lithium battery using a nonaqueous solvent comprising a large amount of ethylene carbonate has poor low-temperature conductivity. To solve this problem, methods for providing a lithium secondary battery having good high-rate discharge characteristics at low temperature have been proposed. Japanese Patent Laid-Open Publication No. Hei 07-153486 discloses a lithium secondary battery using an electrolytic solution comprising 0.5 to 50% by volume of γ-butyrolactone and a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1. Although adding γ-butyrolactone improves high-rate discharge characteristics at low temperature, life characteristics of a battery may deteriorate.
To suppress swelling of a battery due to gases generated when left at high temperature and improve high-capacity discharge characteristics and charge/discharge cycle life characteristics of a battery, U.S. Pat. No. 6,503,657 discloses a nonaqueous electrolyte secondary battery using a nonaqueous solvent comprising 5 to 40% by volume of ethylene carbonate, 40 to 95% by volume of γ-butyrolactone and 0.05 to 10% by volume of vinylene carbonate. However, if the amount of γ-butyrolactone unduly increases, life characteristics of a battery may deteriorate.
To provide a high-capacity nonaqueous secondary battery, Japanese Patent Laid-Open Publication No. Hei 06-20721 discloses a nonaqueous secondary battery comprising a carbonaceous cathode including graphite having d-value (d002) of the lattice plane (002) obtained by X-ray diffraction of less than 0.337, 20 to 50% by volume of γ-butyrolactone and a remainder of cyclic carbonates. However, since the nonaqueous solvent does not contain linear carbonates, the viscosity of an electrolytic solution is high so that the ion conductivity is low, and low-temperature discharge capacity may deteriorate.
To provide a nonaqueous secondary battery having excellent low-temperature characteristics, Japanese Patent Laid-Open Publication No. Hei 08-64242 discloses a nonaqueous secondary battery using a mixed solvent comprising 10 to 50% by volume of γ-butyrolactone and/or sulfolane, and 90 to 50% by volume of dimethyl carbonate. However, the nonaqueous electrolytic solution system of the secondary battery has a low dielectric constant, resulting in poor life characteristics of the battery.
Therefore, it is highly demanded to develop a lithium battery which can exhibit improved charge/discharge cycle characteristics, effective high-temperature storage stability and effective low-temperature discharge characteristics by varying compositions of a nonaqueous mixed solvent used for an electrolytic solution of a conventional lithium battery.